This task could vary from rather simple to extremely difficult depending on the site you wish to convert over. Unfortunately there isn't a tool that will do the work for you. This task involves studying the WordPress codex in detail and inquiring as much as possible with other experienced developers.

It's a rather intimidating task, but the more time you spend in the documentation, the less intimidating the giant appears. I've been running myself over the coals for about a month now, and am working on my third conversion this week. The first two were terrifying, but they've been getting easier and easier.

Firebug > Edit HTML > Strip out the content then save as source. You can then just drop in your WP snippets. At the very least, you'll have a starting point. I wouldn't recommend this method for a paying client, but it's quick and dirty and can be done.